Investment casting method and pattern material comprising thermally-collapsible expanded microspheres

ABSTRACT

A method and composition for forming investment casting patterns wherein thermally-collapsible microspheres are incorporated into the pattern composition. The patterns can be made either by conventional pattern forming techniques or by solid imaging techniques. After investing the microsphere containing pattern in the ceramic shell, the pattern and shell are heated, causing a collapse of the microspheres and thereby preventing cracking of the shell. The shell is then heated to burn-out the remaining pattern material and fire the shell, thereby creating a mold.

FIELD OF THE INVENTION

This invention relates to production of investment casting patternscomprising expanded thermally-collapsible microspheres and moreparticularly to a method of reducing ceramic shell mold failures due tocracking from pattern expansion during pattern melt-out or burn-out andfiring of the mold.

BACKGROUND OF THE INVENTION

Investment casting by the "lost wax" process was first known to be usedby the Egyptians and still finds practical utility today. The processinvolves many steps which may be summarized as follows:

a. Mold, carve, or machine a wax pattern.

b. Dip the wax pattern in a series of ceramic slurries in order tocreate a ceramic shell which is allowed to dry somewhat around the waxpattern.

c. Heat the ceramic shell and wax pattern in an oven or an autoclaveuntil the wax melts and evacuates the ceramic shell.

d. Fire the ceramic shell.

e. Pour a melted alloy into the ceramic shell and allow it to solidify.

f. Break the ceramic shell to obtain the solidified alloy in the shapeof the wax pattern.

Using investment casting techniques, parts may be molded having shapesthat could not be molded by other techniques. Investment casting alsooffers the potential for manufacture of parts made from many differentalloys. Currently, some of the problems involved with the investmentcasting process particularly related to the wax pattern are as follows

a. Production of a wax pattern is time consuming if made by conventionalmachining techniques, and if the wax pattern is molded, shrinkage of thewax upon cooling creates loss of tolerances due to uneven solidificationrates as it cools to a solid.

b. The wax is sometimes brittle and easily damaged before and during theceramic slurry coating process. In addition the drying of the ceramicshell must be conducted under tightly controlled temperature conditions,since an increase in temperature of the wax pattern during the shelldrying process may crack the shell due to pattern expansion. Also theceramic slurry must easily wet the wax pattern to form a good mold thatfaithfully represents the pattern's shape and surface finish.

c. During the wax pattern removal stage, the wax must melt in such amanner as to not cause cracking of the mold due to the expansion of thepattern as it heats up.

d. During the ceramic shell firing, any wax that remains or has soakedinto the shell must burn-off leaving very little residual ash.

Solid Imaging, or the direct production of objects or mold patterns fromcomputer aided design data, holds promise to solve several of theproblems mentioned above relative to use of wax patterns for investmentcasting. Many patents have issued in the field of solid imaging thatdescribe the technologies' potential use in investment casting, butprior to the invention of a photoformable composition as describedherein, no photoformable composition had been developed that would solvein particular the above described shell cracking problem.

In a patent assigned to DeSoto, Inc., Des Plaines, Ill. (U.S. Pat. No.4,844,144, Jul. 4, 1989) there is proposed the use of a photosensitiveethylenically unsaturated liquid which solidifies to a thermoset shapeupon exposure, in mixture with primarily a substantially inert lowtemperature thermoplastic material which becomes partially solidified orsomewhat bound within the photosolidified thermoset matrix In thispatent and in a publication "Investment Casting of Optical Fabricationand Stereolithography Models" by Myron J. Bezdicek of DeSoto (publishedby the Investment Casting Institute, 1989, 37th Annual TechnicalMeeting) it is proposed that the thermoplastic component of thephotopolymer composition softens during melt-out or burn-out of thephotoformed pattern and thereby weakens the thermoset structure of thepattern lessening the effects of expansion during subsequent heating.Although this approach is believed to provide an improvement whencompared to the use of other photosensitive compositions to producepatterns for investment casting purposes, mold cracking still occurs andfurther improvements are necessary. The author and inventor proposeother fixes such as block or flask casting, wax coating, andhollow-walled patterns, but for other practical reasons these fixes arenot always desirable approaches for the investment caster.

U.K. Patent Application GB 2207 682 A (published Jun. 8, 1989) disclosesa photosensitive composition, for use as a pattern for dental braces, inwhich one component of the pattern softens prior to burn-out of thepattern thereby preventing the build up of pressures that might causeshell cracking.

A patent application in Japan, which is a "laid open to publicunexamined application" 2-116537 (Matsushita Electric Works), with apublication date of May 1, 1990, describes the advantages of usingexpandable microspheres, balloons, and/or rubber bead fillers, within aphotocomposition to prevent shrinkage and the build-up of stressesduring solid object formation by essentially solid imaging meansAlthough the compositions described, specifically in the relation to theaddition of Expancel® microspheres to the composition, havesimilarities, the application does not discuss the use of thesecompositions for the production of investment casting patterns, and theapplicants apparently were unaware of the rather surprising fact thatthe microspheres and/or balloons could be thermally collapsed inaddition to being thermally expanded In addition, the applicationdescribes only ultrasound methods of keeping particles uniformly in adispersion.

Other pertinent art is disclosed in U.S. Pat. No. 3,822,138 (Norguchi etal.) in which carbon microspheres are added to a wax pattern compositionin order to reduce pattern shrinkage during pattern composition coolingin the mold. Although, such carbon microspheres may decrease patternshrinkage during molding, they are not added to allow thermal collapseand reduction of pressures during pattern melt-out or pattern burn-outsteps in an investment casting process as disclosed herein by theinstant invention.

A published unexamined Japanese Application Kokai: 30 50-7722 (PublishedJan. 27, 1975) describes a method by which surface blisters in a moldedwax pattern can be reduced by mixing phenol-based or vinylidenechloridebase fine hollow resin spheres in the wax prior to molding. Thepublication describes excellent ways to mix and prepare the wax/hollowsphere compositions, however, the publication does not describe anyadvantages relative to the reduction of shell cracking during themelt-out and firing stages of mold production. Typically thephenol-based microspheres are thermoset and therefore notthermally-collapsible as described in this disclosure. And typically thevinylidene-chloride based microspheres, which generally have a glasstransition temperature on the order of -15° C., have higher diffusionrates at room temperature. This greater diffusion rate would allow theblowing agent used to create the microsphere to escape, causingpremature collapse of the spheres or replacement of the blowing agentwith other gasses. The production of copolymer vinylidene-chloride andacrylonitrile hollow microspheres, which have lower diffusion rates atroom temperature, was very difficult in 1973 when the above Japaneseapplication was filed. However, in February 978 the Dow Chemical Companydisclosed (U.S. Pat. No. 4,075,138, J. L. Garner) a method of makingcopolymer vinylidene-chloride and acrylonitrile hollow microspheres,which contained an isobutane blowing agent, in production quantities. Itis these, or very similar, microspheres, herein disclosed, that arecomprised in the pattern compositions of this invention and that aid inthe reduction of shell mold cracking during the investment castingprocess.

Other pertinent art is disclosed in U.S. Pat. No. 4,790,367 (Moll etal.) wherein a foamed pattern is produced, coated with a silica slurry,packed in sand, and burned out using molten metal. In essence theprocess, which is termed a "Lost Foam" or "Lost Plastic" process,consists of the following steps:

1. Prepare the Plastic Material: By preparation of the plastic material,beads, typically consisting of PMMA or other resins containing anunexpanded blowing agent, are produced

2. Pre-expand the beads to a loose packed density of around 10% greaterthan used in the molded form.

3. Age the beads. This usually involves drying the pre-expanded beads.

4. Mold the beads. In this step the beads are pneumatically loaded intothe mold and steamed, causing them to bond together into a foamedpattern.

5. Age the molded pattern. This typically involves drying the pattern.

6. Assemble the pattern parts.

7. Refractory coat the pattern. The refractory coating provides asmoother finish than would be given by the packed sand (from step 9below) and helps contain the molten metal during the casting step.

8. Attach the gates, runners and sprues

9. Pack the pattern, gates, runners and sprues with sand, which isvibrated to create greater compaction.

10. Pour the casting. In this step, the foam resins are effectivelythermally degraded to lower molecular weight volatile components whichare essentially burned out of the packed space and replaced by themolten metal. This disclosure describes methods and compositions toreduce carbon residue during this burn-out process.

11. Cool the casting.

12. Remove the sand from around the casting.

13. Clean the casting. U.S. Pat. No. 3,942,583 (Baur) discloses asimilar "Lost Plastic" process in which foam pieces, such as foamplates, foam beams, foam columns, etc., are assembled together, thensand packed, and burned out by the pouring of the molten metal. AlsoU.S. Pat. Nos. 4,773,466 (Cannarsa et al.) and 4,763,715 (Cannarsa etal.) disclose a "Lost Foam" process utilizing alternative resins to formthe foam beads. Although such art involves heat destruction of foamedpatterns, the art does not disclose the reduction of shell mold crackingin an investment casting application utilizing thermally-collapsiblemicrospheres mixed in a pattern composition.

Another U.S. Pat. No. 4,854,368 (Vezirian) discloses the added step ofapplying a vacuum during heat removal of a foamed pattern prior tofilling the invested mold with metal. The added vacuum preventsdecomposition products from entering the atmosphere. Other disclosures,such as U.S. Pat. No. 4,891,876 (Freeman), U.S. Pat. No. 4,830,085(Cleary et al.), U.S. Pat. No. 4,787,434 (Cleary et al.), U.S. Pat. No.4,520,858 (Ryntz, Jr. et al.), and U.S. Pat. No. 3,426,834 (L. J. Jacobset al.) describe various other modes and methods of conducting "LostFoam" casting to make various objects.

In a patent assigned to Spectra-Physics, Inc., San Jose, Calif. (U.S.Pat. No. 4,915,757 Apr. 10, 1990) there is described a process wherebysmall balls, microspheres, grains of sand, etc., initially bondedtogether by a wax or a weak cement within a block, are removed inselective regions by laser heating or blasts of impinging hot air inorder to form three-dimensional models. No specific mention is made asto the utility of these models for investment casting purposes, andindeed the material of the balls does not appear to make the modelsproduced by this method useful for investment casting applications,since there does not appear to be consideration of the thermal expansioncharacteristics required and the burn-out characteristics necessaryrelative to the materials proposed.

Also published by the Investment Casting Institute, 1989 at the 37thAnnual Technical Meeting is a paper "Applications of Stereolithographyin Investment Casting" by Frost R. Prioleau, of Plynetics Corporation,in which a "proprietary process" using stereolithography patterns forinvestment casting is mentioned. This process apparently reducescracking of most shells, however, the need for new photopolymers withbetter "removability" is also stated.

An excellent insight into "The Causes and Prevention of Shell MouldCracking" a materials and processes committee report is published by theBritish Investment Casting Trade Association (August 1975, Royton House,George Road, Edgbaston, Birmingham, B15, INU). This publicationdescribes the effects of pattern wax on the mold shell during thevarious production steps and especially suggests that during the waxpattern melt-out from the ceramic shell mold, waxes should be chosenthat have low thermal conductivity and high permeability into theceramic shell. The thinking is that during the melt-out step, heat istransferred from the ceramic shell to the outer layer of the wax patterncausing the outer layer to melt. If just the outer wax layer meltsfirst, without a substantial amount of heat being conducted into theinner region of the wax pattern, which would cause expansion and shellcracking, and if the outer wax layer can flow out of or permeate intothe ceramic shell quickly enough, then shell cracking will be minimized.

It may be that shell mold cracking using the process suggested by DeSotostill occurs since, although the thermoplastic component of theirpatterns softens, there is still substantial resistance to flow orescape of this melted component due to constraint within thephoto-thermoset matrix.

SUMMARY OF THE INVENTION

In accordance with this invention, a method of investment casting isprovided which utilizes thermoplastic hollow thermally-collapsiblemicrospheres within the pattern composition. The pattern is formed frommaterials comprising thermoplastic hollow thermally-collapsiblemicrospheres by conventional means, using an investment casting patternwax, or by solid imaging means, using photoformable compositions.Afterwards the pattern is coated with a series of ceramic slurries usingconventional investment casting techniques. Upon heating of the castmold and pattern, bringing portions of the pattern to an elevatedtemperature, the thermoplastic hollow thermally-collapsible microspherescollapse, thereby reducing the buildup of pressures within the mold andavoiding a tendency for the mold to crack. Upon further heating, thepattern material is burned-out from the ceramic mold and the mold isfired. Finally, the ceramic mold is filled with an alloy to cast a partusing conventional investment casting methods.

Therefore, herein is proposed a method of forming an investment castingmold comprising the steps of:

(a) placing a photoformable composition comprising thermally-collapsiblemicrospheres in a vat;

(b) forming a pattern by solid imaging means;

(c) attaching said pattern to a gate and sprue, formed from a wax, inorder to create a pattern cluster;

(d) dipping said pattern cluster into ceramic slurries in order tocreate a stucco shell of ceramic layers;

(e) heating said stucco shell and said pattern cluster to a temperaturehigh enough to collapse said microspheres, melt said gate and sprue, andallow said wax to substantially drain from said stucco shell; and

(f) firing said stucco shell in order to burn off the pattern and sintersaid stucco shell in such a manner as to form the casting mold.

Furthermore, herein are proposed compositions for an investment castingpattern comprising:

(a) a monomer

(b) a photoinitiator; and

(c) thermally-collapsible microspheres.

DETAILED DESCRIPTION OF THE INVENTION

As described in the summary, the present invention encompasses a methodof investment casting wherein the pattern material, i.e., the materialwhich is used to shape the interior contours of a ceramic mold usefulfor investment casting purposes, comprises thermally-collapsiblemicrospheres as one of its components. The purpose of thethermally-collapsible microspheres is to provide a means of contractionof the pattern when the pattern is being melted or burned out of theceramic mold.

The presently preferred thermally-collapsible microspheres aremanufactured by Expancel (Nobel Industries Sweden, Sundsvall, Sweden).The Expancel® microspheres consist of isobutane gas surrounded by acopolymer of vinylidene chloride and acrylonitrile shell. Thesemicrospheres can be obtained in unexpanded form and, as most preferredfor the purposes of this invention, in expanded form. The vendor iscapable of supplying the expanded microspheres, which are available inthe range of 10-100 μm, in an expanded state. The plastic shell of theexpanded microspheres is very thin, but the microspheres still retainconsiderable strength and will return to the expanded shape even afterbeing compressed. Upon heating these expanded microspheres to atemperature, ranging typically above 124°-154° C., the microspheresrelease the isobutane gas and collapse.

It should be possible to utilize such expanded thermally-collapsiblemicrospheres in conventional pattern waxes used in the investmentcasting industry. Typically wax patterns are made by pouring orinjecting a wax, which is just at or slightly above its melting point,into a mold. Pouring the wax near its melting point is preferred sincethe pattern solidifies faster and there is less shrinkage duringsolidification. The melting point of most waxes is on the order of49°-89 ° C., which is well below the collapse temperature of themicrospheres. And the ability of the thermally-collapsible microspheresto withstand compression would allow a pattern composition comprisingthermally-collapsible microspheres to be injected into the mold. Also,since the microspheres might comprise, for example, 25-30% of thepattern material volume, and the expanded thermally-collapsiblemicrospheres exhibit little volumetric change upon, for example, coolingfrom a wax melt temperature to room temperature, it would be expectedthat the wax pattern as a whole would exhibit reduced shrinkage andtherefore improved dimensional control.

Although the expanded thermally-collapsible microspheres might separatefrom the wax melt due to the marked difference in specific gravity (theexpanded microspheres have a true density on the order of 30-50 kg/m³),agitation of the wax in the melt state prior to molding (a commoninvestment casting practice in order to maintain even temperaturedistribution while heating the wax), the use of specialized surfactants,or maintaining the wax in a pseudoplastic (shear-thinning) or toothpastelike condition (as is preferred for injection of the wax into the mold)would substantially diminish the separation of the expandedthermally-collapsible microspheres from the wax.

Use of the expanded thermally-collapsible microspheres in the patternwax composition should not affect the surface finish of the patternafter molding since the wax would fill in the regions between themicrospheres and the mold walls. However, if expandedthermally-collapsible microspheres are comprised in a wax patternmaterial that is machined to the pattern shape, a change in surfacefinish of the pattern might be expected, especially if larger particleexpanded thermally-collapsible microspheres are utilized. In such casesit might be preferred to use, for example, Expancel® 551 WE (or DE) 20microspheres which have an average particle size of 10-20 μm.

Waxes that may be used in the thermally-collapsible microspheredispersion are ozocerite waxes, petroleum waxes, rosin filled waxes, orany of a number of waxes with the possible exception of cranberry waxwhich has a melting point of over 200° C.. There are modern blends ofsuch waxes that are commonly use in investment casting which comprisenumerous components such as natural hydrocarbon waxes, natural esterwaxes, synthetic waxes, synthetic and natural resins, organic fillermaterials and water. Hydrocarbon waxes, natural ester waxes, syntheticwaxes and the resins used are usually compounds of straight chainedcarbon atoms but they could also be ring structured carbon atoms. Themajor criteria in the use of a wax are its shrinkage upon cooling,melting point, residual ash, hardness when solid, flowability, andviscosity (kinematic and dynamic).

Expancel® is provided, by the vendor, with specific handling, pumping,and mixing instructions. Generally however, the appropriate amount, e.g,25-30% by volume of wax, dry form of Expancel® expanded microsphereswould be transferred from the bag using vacuum to the mixing tank. Thenthe mixer would force the expanded microspheres down into the melted waxuntil the pattern composition is thoroughly mixed. Care should beexercised to ensure that the mixture temperature is substantially belowthe expanded microsphere collapse temperature. If unexpandedthermally-collapsible microspheres are first mixed in the wax, it shouldbe possible to expand the spheres to a maximum size by raising thetemperature to approximately the lowest T(max) by a process recommendedby the vendor. The amount of unexpanded thermally-collapsiblemicrospheres should be based however on the final volume mix desired forthe wax pattern material when the microspheres are fully expanded.

Alternatively and more preferably, the pattern may be made utilizingsolid imaging technology wherein the pattern material comprises theexpanded thermally-collapsible microspheres within a photoformablecomposition. For example, the following photoformable composition wasproduced:

EXAMPLE 1

    ______________________________________                                        Component              % by Wt.                                               ______________________________________                                        Photomer ® 4127    8.3                                                    (propoxylated neopentylglycol                                                 diacrylate, Henkel Corporation,                                               La Grange, IL)                                                                V-Pyrol ®/RC       24.5                                                   (N-vinyl-2-pyrrolidone                                                        GAF Chem. Corp., Wayne, NJ)                                                   Plasthall ® 4141   17.2                                                   (triethylene glycol dicaprate,                                                triethylene glycol dicaprylate                                                CP Hall Company)                                                              Ebecryl ® 3704     23.6                                                   (Bisphenol A bis(2-hydroxypropyl)                                             diacrylate, Radcure Specialties Inc.,                                         Louisville, KY)                                                               Ebecryl ® 3604     17.2                                                   (rubber-modified acrylated                                                    epoxy oligomer,                                                               diluted with 20% tripropylene                                                 glycol diacrylate                                                             Radcure Specialties Inc.,                                                     Louisville, KY)                                                               Irgacure ® 651     1.6                                                    (2,2-dimethoxy-2 phenylaceto-                                                 phenone, CIBA-Geigy Ltd.,                                                     Switzerland)                                                                  Elvacite ® 2041    1.0                                                    (polymethyl methacrylate                                                      Du Pont, Wilmington, DE)                                                      PPC-0100 ®         5.1                                                    (polypropylene carbonate                                                      ARCO Chemical Co.,                                                            Newtown Square, PA)                                                           Expancel ® 461DE   1.5                                                    (Expancel, Sundsvall, Sweden)                                                 ______________________________________                                    

The three low-viscosity liquids, Photomer® 4127, V-Pyrol®/RC, andPlasthall® 4141 were mixed together, and the solid polymers, Elvacite®2041 and PPC-0100®, were added and stirred at 120° F. until dissolved.After cooling to room temperature, the Ebecryls® and Irgacure® wereadded and stirred until dissolved. Finally the Expancel® was added andstirred rapidly to disperse the expanded thermally-collapsiblemicrospheres in the mixture. The stirring rate was then dropped to a lowrate (50 rpm) to allow air bubbles to dissipate while maintaining themicrospheres in suspension. The mixture was kept stirred at this lowrate until used for making the patterns.

Although the above formulation comprises only 1.5% by weight ofExpancel® 461DE it comprises about 25-30% of the microspheres by volume.These microspheres may tend to come out of dispersion, due to thedifferences in specific gravity between the microspheres and theformulation remainder, unless mixed frequently. Mixing of the dispersionshould be performed on the order of two to three times a day.

The above photoformable liquid formulation was placed in a vat andexposed with a focused UV laser beam, which was scanned in selectiveregions on the surface of the liquid thereby hardening those selectiveregions and creating a layer which represented a cross-section of thethree-dimensional pattern. During this exposure, the first layer becameattached to a platform that was positioned within the liquid one layer'sthickness below the surface of the liquid. Although leveling of aphotoformable liquid surface largely relies on natural flattening of thesurface by surface tension effects, the application of the photoformablecomposition in layers in this case was accomplished by first dipping along dispenser with a slot into the photoformable composition, raisingthe dispenser filled with composition above the composition surfacelevel, and allowing the composition to flow from the dispenser throughthe slot while traveling in front of a doctor blade that smoothed thecomposition to the proper layer thickness above the platform or aprevious photoformed layer. Once this first layer was formed, theplatform was translated deeper into the liquid a distance of one layerthickness and a second layer of photosensitive liquid was formed on topof the previous layer. This liquid was then exposed selectively,creating another hardened layer, which represented the nextcross-section of the three-dimensional pattern, and which attached tothe surface of the previous layer. The process of platform movement,forming a liquid layer, selective exposure, etc. was continued until athree-dimensional pattern was produced.

Patterns produced, by solid imaging means, for testing the utility ofthis photosensitive formulation for investment casting purposes weremade from 10 mil thick layers and were approximately three inches by twoinches by a third of an inch high. There are no indications that thereis any limitation to the size of pattern that may be utilized or made bythis process. And there are no indications that thick sections of thepattern will pose any particular problems. Therefore, it should not benecessary to produce hollow patterns or use special procedures toprevent shell cracking during the casting process, though suchprocedures may be used.

In the case described the laser beam was scanned across the surface ofthe vat in a manner similar to that described in the Hull (U.S. Pat. No.4,929,402) with the exception that exposure control was utilized tomodulate the beam spot in the image plane. Effectively, with exposurecontrol, the laser beam is modulated digitally corresponding to adiscrete distance that the beam has moved in the image plane. Since themodulation of the laser beam calls for the laser beam to be on for aspecific time frame per digital pulse, the result is a substantiallymore uniform exposure, and more uniform depth and width ofphotoformation, per distance moved by the laser beam in the image plane.However, it is not necessary to provide the exposure by use of a laser.The exposure could be made by, for example, UV light exposure through orreflected from an appropriate photomask, or, other radiative exposuremethods, such as, for example, x-rays, microwave or radio-frequency waveexcitation, and the like may be used, assuming such radiation inducesphotoforming of the photoformable composition. Photomasks useful for thepractice of this invention may be silver halide films (eithertransmitted through or backed by a mirror and reflected through), liquidcrystal cells (reflective or transmissive), electrostatically depositedpowders on a transparent web, ruticons, etc.

In this discussion a clear distinction should be made between aphotoformable and a photoformed composition. The former refers to onewhich has not yet been subjected to irradiation, while the latter refersto one which has been photoformed by irradiation. Also a photoformablecomposition may be in the form of, for example, a liquid, a semi-solid,a paste, a non-photoformed solid, or a gel. These photoformablecompositions preferably may exhibit non-Newtonian flow characteristics,as previously mentioned, such as pseudoplastic flow, plastic (Binghambody) flow, and/or thixotropic flow. Or, these photoformablecompositions may be heat liquefiable, as long as the liquified flowtemperature of the composition is below that of microsphere collapsetemperature, such that the compositions are applied in layers in aliquefied state, but after coating and when cooled they solidify or forma near solid. Such compositions would generally remain heat liquefiableuntil exposed or photoformed, in which case they would not liquefy untilmuch higher temperatures, if at all.

The patterns, made by solid imaging means as described above, wereattached to separate wax gates and wax sprues creating a patterncluster, then coated (invested) and dried six times according toconventional investment casting practice in a conventional ethylsilicate based slurry. Typically, the photoformed patterns, made fromthe above photoformable liquid, will have a molecular polarity orsurface energy that is substantially higher than that of the investmentcasting wax patterns. The photoformed patterns will therefore be moreeasily wetted by the slurry during the coating process. For example, aninvestment casting wax produced by Lanxide (Newark, DE) 5550K-GRN-FLKwas compared against the above photoformed pattern in terms of contactangle and surface energy. The surface energy of the wax was determinedto be 35 dynes/cm whereas the photoformed pattern was determined to havea surface energy of 63 dynes/cm. When comparing the contact angle ofboth a polar liquid (distilled water) and a non-polar liquid (methyleneiodide), it was found that the contact angle on the wax wasapproximately 40° higher than the contact angle measured on thephotopolymer. Both measurements indicate that when the ceramic slurry isapplied to the photoformed pattern, there will be improved wetting ofthe pattern surface, and therefore less tendency for voids in the slurrycoating and greater surface definition in the casting and cast part. Itis not necessary to attach the pattern to a wax gate and sprue, indeedsuch a gate and sprue could be fabricated from the thermally-collapsiblemicrosphere containing composition by solid imaging means, however,formation of the gate and sprue in the casting mold is oftenadvantageous for subsequent casting.

Next four samples of the invested, solid imaged, thermally-collapsiblemicrosphere comprising formulation patterns, gates, and sprues weresubjected respectively to four different types of heating and firingmethods commonly used in investment casting practice. Typically the moldmaker will heat the slurry coated and dried pattern cluster to aninitial relatively low temperature, in an autoclave or oven, in order tomelt out the wax sprue, gate, and pattern prior to firing the mold andin order to recover the wax and reduce the emissions. In the case of thesolid imaged patterns made according to this invention, the pattern maynot melt during the low temperature wax removal steps and therefore thewax can generally be recovered. However, the pattern may be burned outduring mold firing.

The four methods of heating and firing the invested samples were asfollows, with heating method b. and firing method d. being morepreferred:

a. Heat in an autoclave at 300° F. and 80 psi. The pressure was broughtup rapidly, in approximately 7 seconds, and maintained for 10 minutes.The wax gates and sprues melted out of the ceramic shell. The shellaround the pattern did not crack.

b. Heat in an autoclave at 300° F. bringing the pressure up slowly, overa 5 minute period, to 80 psi and maintain for 10 minutes. The wax gatesand sprues melted from the shell, without cracking the shell, eventhough this method of wax removal often does crack investment castshells. The shell around the pattern did not crack. The ceramic shelland the pattern was then flash-fired at 1800° F. for two hours,sintering the shell and burning-out the pattern completely. This shellwas later used for casting 17-4 stainless steel. The cast part showedexcellent reproduction of the solid imaged thermally-collapsiblemicrosphere pattern shape and surface texture, indicating that thepattern had burned out cleanly and was not affected the ceramic mold.

c. Heat in an oven slowly raising the temperature over a period of sevenhours with the last two hours at 1800° F. The solid imagedthermally-collapsible microsphere pattern comprising shell did not crackduring heating or firing. After firing the shell was deliberatelycracked open. There was no noticeable ash left from the pattern and theinterior surface of the ceramic mold appeared to have no adverseinteraction with the pattern burn-out.

d. Flash-fire for two hours in a 1800° F. oven. The ceramic shell aroundthe solid imaged thermally-collapsible microsphere comprising patterndid not crack and the mold produced was successfully used to cast a 17-4stainless steel part.

As a control in each of the above heating and firing tests, aphotosensitive composition, similar to that described in the DeSoto U.S.Pat. No. 4,844,144 Table I, was produced and used to form controlpatterns by solid imaging means in substantially the same manner, shape,and size as was used to produce the microsphere comprising patternsdescribed above. These control patterns were attached to similar gatesand sprues, and subjected to substantially the same heating and firingconditions. In every case, the ceramic shell around the control patternscracked.

The Applicants propose the following explanation for the successfulproduction of investment casting molds utilizing thermoplastic hollowthermally-collapsible microspheres comprised in the solid imagedpattern. However, this explanation should only be taken as a suggestionto the reader, and by no means, should the Applicants' explanation beconstrued as limiting in any way the breadth and scope of thisinvention.

During formation of the wax or solid imaged pattern comprising expandedthermally-collapsible microspheres, there is less tendency for shrinkageof the wax pattern during cooling, or of the solid imaged photoformablecomposition during photoforming, due to the presence of themicrospheres, which make up a substantial portion of the volume of thepattern and which substantially do not change in volume during eitherthe cooling or photoforming process.

It is believed that, during the heating of the pattern within theceramic shell, since the thermally-collapsible microspheres aresubstantially insulating, there is a greater temperature differential,between the outer surface of the pattern and the pattern interior, thanthere would be with patterns that do not comprise such insulators. Thatis, the temperature at the microsphere comprising pattern surface mayrise to a relatively high temperature prior to a significant temperaturerise of the pattern interior. Furthermore, the temperature at the outersurface of the pattern is believed to rise above the collapsetemperature of the thermally-collapsible microspheres prior to asignificant temperature rise and therefore expansion of the patterninterior. It is the collapsing of the microspheres and the permeation ofthe gas out of the pattern surface and out of the ceramic shell thatreduces the volume of the pattern and thereby prevents a pressurebuildup within the shell during subsequent heating and expansion of thepattern interior.

In general then, a thermally-collapsible microsphere is a particle whichretains a substantial degree of dimensional stability and resiliencefrom the time of production, through periods of shipping, storage,mixing in with other components of a composition, forming (for example,molding into a pattern shape or photoforming into a pattern shape), andprocessing (for example, attaching to a sprue and gate and investing ina ceramic shell). Yet, the microsphere collapses when heated to atemperature, which temperature is substantially below the degradationtemperature of the surrounding composition and preferably is atemperature low enough that substantial thermal expansion of the patterndoes not occur. And when the microsphere is burned, it preferably leaveslittle or no ash.

The thermoplastic microspheres collapse and subsequent escape of thegas, originally comprised within the expanded thermally-collapsiblemicrospheres, may be aided by a simultaneous melting of the outersurfaces of the pattern, since a softening of the pattern matrix,whether it be a wax or a solid imaged photoformed pattern, would allowthe collapse of the microspheres and would enhance the permeation of thegas through the melted pattern material and through the ceramic shell.The presence of a plasticizer in a solid imaged pattern composition,e.g,. Plasthall 4141 in the above formulation, may aid the microspherecollapse and gas permeation, though a plasticizer may not be necessary.It may also be an advantage to comprise a thermoplastic component,within the solid imaged pattern composition, to aid in microspherecollapse, though it is not necessary. Another advantage to be expectedfrom use of expanded thermally-collapsible microspheres, as part of thepattern composition, is an increase in impact strength in the pattern.This would reduce pattern breakage during handling and coatingoperations.

Surface treatment of the microspheres with surfactants may stabilize thedispersion for longer periods of time. A mixture in which thethermally-collapsible microspheres do not rise to the surface onstanding could be made by substantially increasing the viscosity of thecomposition. For ease of coating during solid imaging, however, it ispreferable to have the composition shear-thin during coating and toincrease in viscosity when not being coated. Such pseudoplastic, or morecorrectly plastic flow behavior, of the photoformable compositioncomprising thermally-collapsible microspheres was achieved by, forexample, incorporating approximately 14-25% by weight of polyethylenebeads (for example Microfine® MF-6X or Eftofine® FT-600 FX from TransPenn Wax Corporation of Titusville, Pa.) into the thermally-collapsiblemicrosphere containing photoformable composition. Such photoformablecompositions have been stable for over three months. Other additives toincrease the shear thinning properties of the photoformable compositionsmay be used, however, there should be consideration of the residual ashof all additive materials during the burn-out and firing steps of moldproduction. For example, an alternative composition using PTFE powderssuch as Fluo 300™ (Micro Powders Inc., Yonkers, N.Y.) would producecompositions with a face-cream like consistency. Although the burn-outtemperature would likely be higher, these powders would still be usefulas an additive in a photoformable investment casting composition.

The addition of 14-25% polyethylene beads to photoformable investmentcasting compositions gave improved stability, however, such compositionswere "paste-like" in low shear rate conditions and not easily coatedwith conventional Solid Imaging coating apparatus. Also, there isconcern that the use of large amounts of fillers may have an adverseeffect on composition photospeed, imaging resolution, or part mechanicalproperties, such as for example, interlayer adhesion. On the other hand,there are organic additives which can be included in the photosensitiveinvestment casting pattern compositions in relatively small amounts.These additives improve the stability of the thermally-collapsiblemicrosphere containing compositions by imparting shear-thinning flowbehavior characteristics. Three examples of such materials are Thixcin®R, Thixatrol® ST, and Thixatrol SR (RHEOX, Inc., Hightstown, N.J.).Thixin® R (trihydroxy stearin) and Thixatrol® ST are powdered organicderivatives of castor oil. Thixatrol® SR is a proprietary mixture of 30%solids in cyclohexanol/petroleum stock. These materials are sold asthixotropes, however, viscosity tests performed with these agents addedto a thermally-collapsible microsphere containing pattern compositionexhibited either pseudoplastic or plastic flow behavior without athixotropic loop. Generally, these agents are added to compositions at0.2 to 0.8% by weight. Though for higher thixotropy index compositions,as much as 2.0% by weight agent may be added.

The following stock solution was formulated:

    ______________________________________                                        Component              % by Wt.                                               ______________________________________                                        Photomer ® 4127    8.7                                                    (propoxylated neopentylglycol                                                 diacrylate, Henkel Corporation,                                               La Grange, IL)                                                                V-Pyrol ®/RC       26.2                                                   (N-vinyl-2-pyrrolidone                                                        GAF Chem. Corp., Wayne, NJ)                                                   Plasthall ® 4141   18.6                                                   (triethylene glycol dicaprate,                                                triethylene glycol dicaprylate                                                CP Hall Company)                                                              Elvacite ® 2041    1.1                                                    (polymethyl methacrylate                                                      Du Pont, Wilmington, DE)                                                      Ebecryl ® 3704     43.7                                                   (Bisphenol A bis(2-hydroxypropyl)                                             diacrylate, Radcure Specialties Inc.,                                         Louisville, KY)                                                               Irgacure ® 651     1.7                                                    (2,2-dimethoxy-2 phenylacetophenone                                           CIBA-Geigy Ltd., Switzerland)                                                 ______________________________________                                    

The first four components of the above stock solution were combined andstirred at 120° F. till well mixed and dissolved, then the remainingcomponents were added and mixed. 150 gram portions of this stocksolution were used in tests for Examples 2-4, in which the followingquantities of the various thixotropes and microspheres were added.

EXAMPLES 2

2 grams of Thixcin® R were added to the stock solution, then mixed in aWaring blender at fast speed for 20 minutes at 120°-130° F. Theviscosity (after the compositions were allowed to rest for enough timeto remove thixotropic effects) was measured using a Brookfield DigitalViscometer, Model DV-II with a #3 spindle from a LV spindle set. (Thespeed of 3 RPM and 30 RPM calculate to a shear rate of 0.63/sec and6.3/sec respectively at the face of the spindle.)

@3 RPM the viscosity was 5,440 centipoise.

@30 RPM the viscosity was 1,420 centipoise giving a thixotropic index of3.8.

Next, 2.25 grams of Expancel® 461 DE were added while stirring with anaverage speed rotor and the following viscosity values were measured:

@3 RPM the viscosity was 12,400 centipoise.

@30 RPM the viscosity was 3,410 centipoise giving a thixotropic index of3.6.

EXAMPLE 3

2 grams of Thixatrol ST were added to another portion of the stocksolution and blended fast at 135° F. for 20 minutes.

Next, 2.25 grams of Expancel 461 DE were added and stirred with anaverage speed of rotor. Using the same equipment as in Example 2 abovethe following viscosity values were measured:

@3 RPM the viscosity was 7,210 centipoise.

@30 RPM the viscosit giving a thixotropic index of 2.9.

EXAMPLE 4

2.25 grams of Expancel® 461 DE were stirred into another portion of thestock solution using an average speed of rotor. Using the same equipmentas in Example 2 above the following viscosity values were measured:

@3 RPM the viscosity was 1,000 centipoise.

@30 RPM the viscosity was 715 centipoise giving a thixotropic index of1.4.

The addition of the microspheres imparts a pseudoplastic or plastic flowcharacteristic to the photoformable composition.

Next 4 grams of Thixatrol® SR were added to the mixture and stirred witha rapid rotor speed for 30 minutes at 120° F. The viscosity valuesmeasured were:

@3 RPM the viscosity was 3,910 centipoise.

@30 RPM the viscosity was 1,970 centipoise giving a thixotropic index of2.0.

Each of the above solutions in Examples 2-4 were placed in separatebrown bottles and examined periodically for separation. The solutionswere rated OK if they appeared to have uniform opacity and NP (notpreferred) if they exhibited a clear layer at the bottom of thesolution. In the case where no thixotrope agent was added, thecomposition tested was that of Example 4 prior to the addition ofThixatrol® SR.

    ______________________________________                                        Solution Age                                                                           No Agent  Example 2 Example 3                                                                             Example 4                                ______________________________________                                        2 hours  NP        OK        OK      OK                                       1 day    NP        OK        OK      OK                                       2 days   NP        OK        OK      NP                                       4 days   NP        OK        OK      NP                                       6 days   NP        OK        OK      NP                                       ______________________________________                                    

The results of this testing suggest, that in solutions containingthermally-collapsible microspheres, that a thixotropic index of at least2.0 is preferred and a thixotropic index of greater than 2.0 is morepreferred. Naturally this conclusion will vary significantly based onthe composition viscosity, in that generally higher viscosity solutions(in low shear-rate conditions) will have less tendency to separate. Buteven with such higher viscosity compositions, a thixotropic index ofgreater than 2.0 is more preferred since coating of solutions ispreferably accomplished under the relatively high shear rate conditionsthat would be imparted by, for example, a doctor blade or a linearextrusion head. It should be clarified that while the term thixotropicindex is utilized, the actual meaning of the index is a measurement ofthe tendency of the composition to shear-thin as a function of shearrate. In the cases sited, the thixotropic index is the ratio of theviscosity at the relatively high shear rate induced by a BrookfieldDigital Viscometer Model DV2 operating at 30 RPM compared to theviscosity measured at 3 RPM. In this sense, no judgement can be made asto whether the compositions are thixotropic, pseudoplastic, or plasticflow (Bingham body) behavioral compositions. However, the compositionsmeasured were clearly shear-thinning as opposed to Newtonian, dilatant,or rheopectic flow behavioral compositions.

It may be possible, in the case of solid imaged pattern compositions tocomprise partially-expanded-thermally-collapsible microspheres in theformulation. In fact, the partially-expanded-thermally-collapsiblemicrospheres are available from the vendor (Expancel). Thesemicrospheres could be, for example, only expanded to 50% of theirmaximum volume and mixed in the remainder of the photoformableformulation. During exposure, a localized temperature rise, created bylight absorption and the heat of photoforming, might be generated suchthat the microspheres would be expanded to a larger volume, but not toso high a temperature as to cause thermal collapse of the microspheres.This expansion of the microspheres during photoforming may effectivelycounteract the shrinkage that normally occurs during photoformingthereby producing a pattern of improved accuracy.

The percentage content of the thermally-collapsible microspheres, byvolume, comprised within either a wax or photoformable composition neednot be just 25-30% as preferred, a range between 5-40% is alsopreferred, and indeed the amount of thermally-collapsible microspheresby volume may be as much as 90% or as little as 1%. The importantcriteria of percent volume concentration within the pattern formulationis dictated by the method of pattern formation and handling requirementsfor the pattern. For example, if the thermally-collapsible microspheredispersion in a wax is formed into a pattern by injection molding or bypour molding means, the concentration of microspheres should be dictatedby the ability of the dispersion to flow into the mold and to accuratelyrepresent the mold contours and surface, and there should be sufficientcasting wax in the dispersion to bond the thermally-collapsiblemicrospheres, and allow handling of the finished pattern without damageduring the subsequent investment casting steps. Likewise, for solidimaging formed thermally-collapsible microsphere containing patterns,the composition should be such that it can be coated in layers necessaryfor the solid imaging process. And there should be enough photoformablecomposition present to bond the thermally-collapsible microspherestogether and allow cleaning and handling of the finished pattern withoutdamage during subsequent investment casting steps. Use of as much as 90%by volume polyethylene beads in a photoformable composition is alsopossible and the same considerations as listed above apply.

A photoformable composition for solid imaging should contain at leastone photoformable monomer or oligomer and at least one photoinitiator.For the purposes of this invention, the words monomer and oligomer havesubstantially the same meaning and they may be used interchangeably.

Examples of suitable monomers which can be used alone or in combinationwith other monomers include t-butyl acrylate and methacrylate,1,5-pentanediol diacrylate and dimethacrylate, N,N-diethylaminoethylacrylate and methacrylate, ethylene glycol diacrylate anddimethacrylate, 1,4-butanediol diacrylate and dimethacrylate, diethyleneglycol diacrylate and dimethacrylate, hexamethylene glycol diacrylateand dimethacrylate, 1,3-propanediol diacrylate and dimethacrylate,decamethylene glycol diacrylate and dimethacrylate, 1,4-cyclohexanedioldiacrylate and dimethacrylate, 2,2-dimethylolpropane diacrylate anddimethacrylate, glycerol diacrylate and dimethacrylate, tripropyleneglycol diacrylate and dimethacrylate, glycerol triacrylate andtrimethacrylate, trimethylolpropane triacrylate and trimethacrylate,pentaerythritol triacrylate and trimethacrylate, polyoxyethylatedtrimethylolpropane triacrylate and trimethacrylate and similar compoundsas disclosed in U.S. Pat. No. 3,380,831, 2,2-di(p-hydroxyphenyl)-propanediacrylate, pentaerythritol tetraacrylate and tetramethacrylate,2,2-di-(p-hydroxyphenyl)-propane dimethacrylate, triethylene glycoldiacrylate, polyoxyethyl-2,2-di(p-hydroxyphenyl)propane dimethacrylate,di-(3-methacryloxy-2-hydroxypropyl)ether of bisphenol-A,di-(2-methacryloxyethyl)ether of bisphenol-A,di-(3-acryloxy-2-hydroxypropyl)ether of bisphenol-A,di-(2-acryloxyethyl)ether of bisphenol-A,di-(3-methacryloxy-2-hydroxypropyl)ether of 1,4-butanediol, triethyleneglycol dimethacrylate, polyoxypropyltrimethylol propane triacrylate,butylene glycol diacrylate and dimethacrylate, 1,2,4-butanetrioltriacrylate and trimethacrylate, 2,2,4-trimethyl-1,3-pentanedioldiacrylate and dimethacrylate, 1-phenyl ethylene-1,2-dimethacrylate,diallyl fumarate, styrene, 1,4-benzenediol dimethacrylate,1,4-diisopropenyl benzene, and 1,3,5-triisopropenyl benzene. Also usefulare ethylenically unsaturated compounds having a molecular weight of atleast 300, e.g., alkylene or a polyalkylene glycol diacrylate preparedfrom an alkylene glycol of 2 to 15 carbons or a polyalkylene etherglycol of 1 to 10 ether linkages, and those disclosed in U.S. Pat. No.2,927,022, e.g., those having a plurality of addition polymerizableethylenic linkages particularly when present as terminal linkages.Particularly preferred monomers are ethoxylated trimethylolpropanetriacrylate, ethylated pentaerythritol triacrylate, dipentaerythritolmonohydroxypentaacrylate, 1,10-decanediol dimethylacrylate,di-(3-acryloxy-2hydroxylpropyl)ether of bisphenol A oligomers,di-(3-methacryloxy-2-hydroxyl alkyl)ether of bisphenol A oligomers,urethane diacrylates and methacrylates and oligomers thereof,coprolactone acrylates and methacrylates, propoxylated neopentyl glycoldiacrylate and methacrylate, and mixtures thereof.

Examples of photoinitiators which are useful in the present inventionalone or in combination are described in U.S. Pat. No. 2,760,863 andinclude vicinal ketaldonyl alcohols such as benzoin, pivaloin, acyloinethers, e.g., benzoin methyl and ethyl ethers, benzil dimethyl ketal;a-hydrocarbon-substituted aromatic acyloins, including α-methylbenzoinα-allylbenzoin, α-phenylbenzoin, 1-hydroxylcyclohexyl phenol ketone,diethoxyphenol acetophenone,2-methyl-1-[4(methylthio)phenyl]-2-morpholino-propanone-1.Photoreducible dyes and reducing agents disclosed in U.S. Pat. Nos.2,850,445, 2,875,047, 3,097,096, 3,074,974, 3,097,097 and 3,145,104, aswell as dyes of the phenazine, oxazine, and quinone classes, Michler'sketone, benzophenone, acryloxy benzophenone, 2,4,5-triphenylimidazolyldimers with hydrogen donors including leuco dyes and mixtures thereof asdescribed in U.S. Pat. Nos. 3,427,161, 3,479,185 and 3,549,367 can beused as initiators. Also useful with photoinitiators are sensitizersdisclosed in U.S. Pat No. 4,162,162. The photoinitiator orphotoinitiator system is present in 0.05 to 10% by weight based on thetotal weight of the photoformable composition. Other suitablephotoinitiation systems which are thermally inactive but which generatefree radicals upon exposure to actinic light at or below 185° C. includethe substituted or unsubstituted polynuclear quinones which arecompounds having two intracyclic carbon atoms in a conjugatedcarbocyclic ring system, e.g., 9,10-anthraquinone,2-methylanthraquinone, 2-ethylanthraquinone, 2-tertbutylanthraquinone,octamethylanthraquinone, 1,4-naphthoquinone, 9,10-phenanthraquinone,benz(a)anthracene-7,12-dione, 2,3-naphthacene-5,12-dione,2-methyl-1,4-naphthoquinone, 1,4-dimethylanthraquinone,2,3-dimethylanthraquinone, 2-phenylanthraquinone,2,3-diphenylanthraquinone, retenequinone,7,8,9,10-tetrahydronaphthacene-5,12-dione, and1,2,3,4-tetrahydrobenz(a)anthracene-7,12-dione; also, alpha aminoaromatic ketones, halogenated compounds like trichloromethyl substitutedcyclohexadienones and triazines or chlorinated acetophenone derivatives,thioxanthones in presence of tertiary amines, and titanocenes.

Although the preferred mechanism of photoforming is free radicalpolymerization, other mechanisms of photoforming apply also within therealm of this invention. Such other mechanisms include but are notlimited to cationic polymerization, anionic polymerization, condensationpolymerization, addition polymerization, and the like.

Other components may also be present in the photoformable compositions,e.g., pigments, dyes, extenders, thermal inhibitors, interlayer andgenerally interfacial adhesion promoters, such as organosilane couplingagents, dispersants, surfactants, plasticizers, coating aids such aspolyethylene oxides, etc. so long as the photoformable compositionsretain their essential properties. The plasticizers can be liquid orsolid as well as polymeric in nature. Examples of plasticizers arediethyl phthalate, dibutyl phthalate, butyl benzyl phthalate, dibenzylphthalate, alkyl phosphates, polyalkylene glycols, glycerol,poly(ethylene oxides), hydroxy ethylated alkyl phenol, tricresylphosphate, triethyleneglycol diacetate, triethylene glycol caprate -caprylate, dioctyl phthalate and polyester plasticizers.

The invention is not limited to the particular embodiments describedabove, but rather is bounded only by the appended claims and their fairequivalents.

What is claimed is:
 1. A method of forming an investment casting moldcomprising the steps of:(a) placing a photoformable compositioncomprising thermally-collapsible microspheres in a vat; (b) forming apattern by solid imaging means; (c) attaching said pattern to a gate andsprue, formed from a wax, in order to create a pattern cluster; (d)dipping said pattern cluster into ceramic slurries in order to create astucco shell of ceramic layers; (e) heating said stucco shell and saidpattern cluster to a temperature high enough to collapse saidmicrospheres, melt said gate and sprue, and allow said wax tosubstantially drain from said stucco shell; and (f) firing said stuccoshell in order to burn off the pattern and to sinter said stucco shellto form said investment casting mold.
 2. A method of forming aninvestment casting mold as recited in claim 1 wherein said microspheresare thermoplastic microsphere shells.
 3. A method of forming aninvestment casting mold as recited in claim 2 wherein said microspherescomprise isobutane.gas surrounded by a copolymer of vinylidene chlorideand acrylonitrile shell.